1. Field of the Invention
The present invention relates to a developing device and an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus including the developing devices, and more particularly relates to a developing device using a developer including toner and carrier.
2. Discussion of the Background
In an electrophotographic image forming method, an electrostatic latent image formed on a latent image carrier is developed with a developer containing a toner. The toner needs to be appropriately charged in the developer to develop the latent image. Generally, there are two methods of developing an electrostatic latent image: (1) a method of developing an electrostatic latent image with a two-component developer including a mixture of toner and carrier, and (2) a method of developing an electrostatic latent image with a one-component developer including toner as a main component.
The developing method using the one-component developer has a disadvantage such as unstable charging property of toner. In the developing method using the two-component developer, a relatively stable good quality image can be obtained. However, deterioration of carrier and variations of the mixing ratio of toner and carrier may tend to occur. When repeatedly developing electrostatic latent images with a two-component developer, a toner density (i.e., a weight ratio of toner to the developer) varies due to consumption of toner in the two-component developer. Therefore, the toner density needs to be controlled by supplying toner to the developer in order to obtain a stable good quality image.
In order to control the toner density, a toner supply control method has been proposed in which a toner supplying device controls the toner supply based on data of a toner density in a developing device. The density is detected by a toner density detecting device using a transmission sensor, a fluidity sensor, an image density sensor, a bulk density sensor, etc. As a recent trend, the image density sensor or a combination of the image density sensor and a magnetic permeability sensor (a kind of the bulk density sensor) is widely used.
In the toner supply control method using the image density sensor, an image pattern formed on a latent image carrier is developed with a two-component developer and exposed to light. A toner supply amount is controlled by detecting the image density of the developed image pattern based on the light reflected from the developed image pattern. In the toner supply control method using the combination of the image density sensor and the magnetic permeability sensor, a toner supply amount is controlled by changing a target value of the magnetic permeability sensor according to the image density of the developed image pattern.
The carrier in the two-component developer includes a core material covered with a resin coating layer. The resin coating layer is used for various purposes such as prevention of toner from forming films on the core material, provision of a uniform, non-abrasive surface, prevention of surface oxidation, prevention of moisture absorption, extension of useful lifetime, protection of a latent image carrier from damages or abrasion by carrier, control of charging polarity, and control of a charging amount. For example, a carrier core material may be coated with a resin material (for example, described in the published Japanese patent application No. 58-108548), or a resin coating layer to which various additives are added (for example, described in the published Japanese patent application Nos. 54-155048, 57-40267, 58-108549, 59-166968, 6-202381, and in the Japanese patent publication Nos. 1-19584, 3-628). Further, additives may be adhered onto a carrier surface (for example, described in the published Japanese patent application No. 5-273789), or a carrier core material may be covered with a resin coating layer containing a conductive powder in which the average particle diameter of the conductive powder is equal to the thickness of the resin coating layer or greater (for example, described in the published Japanese patent application No. 9-160304). Moreover, a carrier coating material may include benzoguanamines-n-butyl alcohol-formaldehyde copolymers as a main component (for example, described in the published Japanese patent application No. 8-6307), or a melamine resin crosslinked with an acrylic resin (for example, described in the Japanese Patent No. 2683624).
Even though a resin coating layer is provided with a core material of carrier, the following problem may arise. When an original document having a low image area (e.g., an occupation ratio of an image on the original document is 3% or less) which subjects a two-component developer to much stresses, is repeatedly printed or copied, the charging amount of carrier increases due to the frictional charging of toner and carrier. As a result, a phenomenon in which a bulk density of the developer decreases due to the repulsive force between carrier particles, may occur. This phenomenon is accelerated when the external agents of toner become embedded in the toner due to rubbing against the toner between the carrier particles, and the fluidity of the entire developer decreases.
The above-described magnetic permeability sensor detects a distance between the magnetic carrier and the sensor. The detected value of the magnetic permeability sensor decreases as the carrier is away from the sensor and as the carrier becomes sparse in the developer. Therefore, when the carrier is away from the sensor and is sparse in the developer due to the decrease of the bulk density of the developer, the detected value of the magnetic permeability sensor decreases, and therefore the sensor erroneously detects that the toner density has increased, although the toner density has not varied. Because the toner supplied to the developer is decreased based on the above detection output of the sensor, the toner density in the developer decreases, thereby deteriorating developing performance. As described above, when the two-component developer is used in a high-stress condition, the bulk density of the developer varies, thereby causing the toner density to be unstably controlled.